Add __ALL__ to all relevant files

This makes `from ggplot import *` cleaner, not cluttering the namespace
with other imported names.
DOC: New documentation system based on numpydoc

The system is stolen from pandas and the current docs are changed
so that they fit into the new doc system.
DOC: Update gitignore with documentation changes
DOC: Make existing docstrings numpydoc compatible
DOC: Use source instead of installed version
Add ggplot.__version__ and use it in setup & docs
Merge pull request #109 from JanSchulz/contributing_additions

Documentation building and some doc additions